1. Field of the Invention
The present invention relates to electric power. More specifically, the present invention relates to selectable compensation of electric power supplies for reactive loads that produce or absorb VARs.
2. Description of Related Art
Power grids transmit electrical energy from generating facilities to end users. Electrical energy is transmitted optimally if the user's loads are resistive. However, the typical load experienced by a power grid is at least partially reactive (inductive or capacitive). For example, a highly inductive load can be produced by a manufacturing facility that has many electric motors. From the standpoint of an electrical grid, reactive loads (VARs) cause problems in transmission of power and can affect the quality of electrical power supplied to the consumer. Furthermore, excessive VARs put undue stress on transmission lines, transformers, and other electrical apparatus.
Power utilities and consumers would both benefit from a device that could adjust the phase of the electrical current to be in phase with the voltage. Such a device would be useful for power utilities, as well as for large electrical users with reactive loads. Furthermore, such a device would protect other electrical users on the grid from power variations that could be destructive.
With reference to FIGS. 1A, 1B, and 1C, electrical power has at least two characteristics: voltage and current. In a typical AC electric grid, both voltage and current vary over time. When the instantaneous voltage is multiplied by the current, the result is the instantaneous power. In a commercial electrical grid, voltage has the form of a sine wave, one cycle of which is illustrated in FIG. 1A. At 60 Hz, the cycle repeats sixty times every second. Ideally, voltage and current waveforms are exactly in phase, as illustrated in FIG. 1A, with v(.omega.t) representing the time-varying voltage waveform and i(.omega.t) representing the time-varying current waveform. However, due to reactive (i.e., inductive or capacitive) loads, the voltage waveform v(.omega.t) may not be in phase with the current waveform as illustrated with reference to FIGS. 1B and 1C. The amount by which the current lags or leads the voltage can be quantified by a power factor angle .phi., which is representative of the fraction of a cycle by which the current leads or lags the voltage. A cycle is 2.pi., or 360.degree., and the power factor angle .phi. is the difference between the cycles of the current and the voltage, such as .pi./2 or 90.degree.. With respect to a constant voltage waveform v(.omega.t), a lagging current (i.e., a current waveform that is behind voltage) is illustrated as I(.omega.t-.phi.) in FIG. 1B, and a leading current (i.e., a current waveform that occurs in front of the voltage) is illustrated as i(.omega.t+.phi.) in FIG. 1C.
FIG. 2 is a phasor representation of an exemplary voltage and current in a circuit. The real components are plotted on the x-axis, and imaginary components are plotted on the y-axis. The voltage vector is shown parallel to the real axis, and the current is leading the voltage by an angle .phi.. The line current, i.sub.line, is the actual current on the line, and it has two components, a real and an imaginary component. The real component of the current is labeled i.sub.active, and is termed the "active current". The imaginary component, labeled i.sub.reactive, is the "reactive current".
The current, voltage, and power have specific relations:
line current * voltage=apparent power (in volt-amperes) PA1 active current * voltage=real power (in watts) PA1 reactive current * voltage=reactive power (in VARs)
Thus, it can be seen that, in the presence of reactive impedances, the product of voltage and current does not indicate the real power used by the circuit. The measured power is "apparent power", and is usually expressed in terms of volt-amperes or "VARs" for volt-ampere reactive. An additional term, a "power factor" is often used for calculation and descriptive purposes. To calculate the real power, which is measured in watts, the apparent power can be multiplied by the power factor. The power factor may be defined as the ratio between the true and apparent power: ##EQU1##
The power factor may also be defined as the ratio between the active current and the line current: ##EQU2##
Practical circuits have a power factor between 0 and 1 (i.e., a power factor .phi. between 0.degree. and 90.degree.). A purely reactive circuit will have a power factor of 0 (a power factor angle .phi. of 90.degree.), and a purely resistive circuit will have a power factor of 1 (a power factor angle .phi. of 0.degree.). Either the power factor or the power factor angle .phi. of a circuit indicate the relationship between the real power and the apparent power.
The power factor angle .phi. between the voltage and current is important from the standpoint of power delivery. From the perspective of a load, power transfer is most efficient when the voltage is in phase with the current. Conversely, from the perspective of a grid, the amount of power that can be delivered into a number of distributed loads is dependent upon the power factor angle between the voltage and current for each of those respective loads. The power factor angle .phi. will be zero, as illustrated in FIG. 1A, only in the unusual instance when the loads are purely resistive. In that instance, all power is being delivered to the load. If the load includes reactive impedances such as inductances or capacitances, then a phase shift will occur.
Compensation of reactive loads can be accomplished conventionally by capacitor banks. However, such capacitor banks have disadvantages: they are large and expensive to build, they generally serve no purpose other than reactive load compensation, and they provide a fixed, non-selectable amount of VARs. In addition to capacitor banks, another device for reactive load compensation is the rotating synchronous condenser, which compensates for a reactive load by operation at a leading power factor. A rotating synchronous condenser can provide a variable amount of VARs, however at a considerable expense in cost.
Other static VAR compensators have been proposed for reactive power correction in a static mode of operation. For example, phase shifting filter circuits may be used to change the power factor angle .phi.. As another example, Gyugyi et al. in U.S. Pat. No. 3,999,117 discloses a static VAR generator circuit for three phase AC that generates time delayed firing angles for thyristor controlled inductors which are utilized with parallel capacitors. Specified phase angles can be maintained; for example it is stated that a phase angle of zero is possible.
Although controlling the power factor angle .phi. is useful in some applications, it does not automatically provide a fixed number of VARs. As an example, FIG. 2 shows a real voltage vector having a length v.sub.1. With the power factor angle .phi..sub.1, r.sub.1 represents the amount of VARs drawn from the line. If the real power were to be changed to be w.sub.2 while maintaining the power factor angle .phi..sub.1, then r.sub.2 represents the amount of VARs drawn from the line. Thus, the effect of a reduction in watts is a direct reduction in the number of VARs at the output. In order to maintain a constant number of VARs, it would be necessary to change the power factor angle to the power factor angle .phi..sub.2, shown on FIG. 2. Thus, as the real power changes, the power factor angle .phi. must be changed in order to maintain a constant number of VARs. In this case, the change in real power from w.sub.1 to w.sub.2 must be accompanied by a power factor angle change from .phi. .sub.1 to .phi..sub.2 in order to maintain the constant number of VARs r.sub.1.
Some proposed static VAR compensators include inverters for converting from a DC power source into a constant frequency alternating current (AC) such as that supplied by a utility grid. In those static VAR compensators, the inverter is based on technology using the silicon controlled rectifier (SCR), a type of thyristor. These static VAR compensators are able to control reactive power and real power to some extent. However, SCR based inverters have problems, including a comparatively low switching frequency resulting in a high harmonic content, and an inherent characteristic that renders them impossible to shut off unless the device voltage is reversed. Furthermore, the SCR-based inverters are generally voltage controlled and as a result, undesirable current transients appear that may last for seconds or more. These and other problems make it difficult to design an SCR-based inverter that can accurately produce the desired waveform without substantial distortion.
Another type of proposed static VAR compensator includes a cycloconverter, which converts between a variable frequency AC and a constant frequency AC. Problems with the cycloconverter include the undesirable current transients that result from the voltage control, as in the SCR based technology discussed above, which may last for several seconds. Also, cyclconverters generate low order harmonics which are difficult to filter out. As an additional disadvantage, cycloconverters require a very complicated control scheme and a large number of semiconductor devices which translate into increased expense and increase the probability of breakdowns and therefore decrease reliability.
Neither the SCR-based inverters, nor the cycloconverters can control the instantaneous current on each phase line; that is, neither can directly regulate the current on an instantaneous basis. Therefore, it is difficult for those devices to provide the necessary relationship between the real current, the reactive current, and the voltage on the power grid. Instead of controlling current directly, these devices are forced to control voltage quantities in the phase relationship between the voltage quantities and only indirectly achieve their goal of controlling the current. It would be an advantage to provide a static VAR compensator that can directly control the current, in order to provide instantaneous current control in accordance with the desired current, in order to produce the appropriate balance between real power and reactive power.
It would be a further advantage is the reactive power could be selected independently of the real power, so that reactive power can be provided while at the same time utilizing only a minimum of real power. Furthermore, it would be an advantage if such a static VAR compensator could alternatively control the power factor angle, which is a combination of the real power and the reactive power in a particular ratio.